For example, in automotive industry, it has been studied to make a structural member absorb an impact energy to enable a vehicle body structure capable of reducing injury for passengers at a collision time. There is a front side member in major structural members which is made absorb the impact energy at a full-lap collision and an offset collision of an automobile. The front side member is made by molding a member by a press molding and so on, and thereafter, closing a section of the member by a spot welding. Normally, the front side member is buckled to thereby absorb the impact energy. It is important to stabilize a buckling mode and to prevent a bent and a fracture in a middle thereof to improve the absorption of the impact energy.
It is necessary to optimize a spot welding interval, a nugget diameter, and welding conditions to stabilize the buckling mode of the structural member. Therefore, a method to find an optimum condition to stabilize the buckling mode by estimating a fracture limit at a spot welded portion from the spot welding interval, the nugget diameter, and the welding conditions has been studied (for example, refer to Patent Literatures 1 to 3).